1. Field of Invention
This invention relates to the use of biological processes to remove paint and comestible polymeric coatings from metallic surfaces. In particular, this invention is directed to the biological removal of organic coatings, especially paint or epoxy resins from beverage cans.
2. State of the Art
Today, many food products, especially beverages (e.g. beer and soda), are distributed in metallic containers coated with paint, epoxy resin, or comestible polymeric coating on the interior and exterior surfaces. Aluminum is the predominant type of metallic container for such use. Each year billions of aluminum beverage cans are produced. Within the past several years, it has become necessary to recycle these cans in order to preserve the environment and the earth's limited resources.
Every year over 600,000 metric tons of these aluminum cans are recycled. Heretofore, containers have been incinerated to remove the paint or other coating before recycling. Incineration requires a large investment of energy to remove the coatings from the metal. Furthermore, by incinerating the cans for recycling, the painted or epoxied surfaces of the cans have presented significant environmental risks since incineration can cause the release of toxic materials such as dioxin and other harmful substances into the earth's atmosphere.
Besides the polluting effects of incineration, significant amounts of aluminum are lost to the atmosphere during such processing, causing further pollution and wasting some of the aluminum which could otherwise be recycled. The loss of aluminum ultimately results in an increase in mining which is wasteful of the nation's resources.
It would be highly advantageous to have an energy efficient, low cost, and environmentally safe way to remove coatings from the surface of beverage and other types of metallic containers prior to recycling in order to lessen the pollution of our environment.
Biodegradation of paints and polymers, such as polyurethane, polyesters and polyvinyls, by microorganisms has been reported in the literature. The effect of bacteria on polymers has been reported by Klausmeier, et al. in Economic Microbiology, Vol. 6, pp. 441-445 (Academic Press 1981) (discussing the effect of Pseudomonas aeruginosa and Cladosoorium resinae on polyurethanes); Pankhurst, et al. in "Investigations Into the Effects of Micro-organisms On PVC Pressure-Sensitive Adhesive Tape and Its Constituents", "The Proceedings of the Fourth International Biodeterioration Symposium-Berlin", Biodeterioration pp.302-16 (Pitman Publishing Ltd., London and The Biodeterioration Society, 1980) (discussing effects of bacteria on PVC product); and Pankhurst, et al. in "The Ability of Polymers Or Materials Containing Polymers To Provide A Source of Carbon For Selected Microorganisms", "The Proceedings of the Fourth International Biodeterioration Symposium-Berlin", Biodeterioration, pp.76-90 (Pitman Publishing Ltd., London and The Biodeterioration Society, 1980). Osmon, et al., in "Rate Limiting Factors In Biodeterioration of Plastics" "The Proceedings of the Fourth International Biodeterioration Symposium-Berlin", Biodeterioration, pp. 66-75 (Pitman Publishing Ltd., London and the Biodeterioration Society, 1980), suggests a mechanism of bacterial biodegradation of PVC or other plastic polymers, and Sharpe, et al., "A Rapid Test For Biodegradability By Pseudomonas Organisms" "The Proceedings of the Fourth International Biodeterioration Symposium-Berlin", Biodeterioration, pp.233-37 (Pitman Publishing Ltd., London and The Biodeterioration Society, 1980) suggests a methodology for determining bacterial biodegradation of PVC film through detection of ammonium ion increase.
Biodegradation of coatings applied to beverage cans has not been previously accomplished. The constituents of the coatings applied to aluminum beverage cans are generally unpublished due to the proprietary nature of the coatings. However, it is known that epoxy resins are a large component of these coatings. Epoxy resins have, to date, been shown to be resistant to microbial degradation, or even inhibitory of microorganismal growth. See, Pankhurst et al. "The Ability of Polymers Or Materials Containing Polymers To Provide A Source of Carbon For Selected Microorganisms", "The Proceedings of the Fourth International Biodeterioration Symposium-Berlin", Biodeterioration, pp.76-90 (Pitman Publishing Ltd., London and The Biodeterioration Society, 1980). "Epoxy resins", as used herein, include paints of which a component is epoxy resin.
"Comestible polymeric coatings", as used herein, refers to those substances typically used as coatings on metallic containers in which food or beverage will be placed so that the food or beverage contained in the container does not react with the metal of the container to cause degradation of the food product, adulteration of flavor, and general contamination of the food. Such materials used for coating metallic containers may be either thermoset or thermoplastic, including phenoxide resins, polypropylene, polyacrylamide, nylon, acrylic resins, and especially epoxy resins. A "coated" surface, as used herein, includes a painted surface, and "coatings" include paints an comestible polymeric coatings: "Paints" are coatings having pigmentation.